The present invention relates most generally to vertical cavity surface emitting lasers (VCSELs). More particularly, the present invention is directed to an apparatus and method that utilize refracted ray coupling principles to monitor the optical output of a VCSEL.
VCSELs have become preferred in today""s optoelectronics industry because they are efficient, small in size, readily assembled into arrays, and easy to manufacture. Furthermore, VCSELs are exceptionally reliable due to good VCSEL-to-VCSEL uniformity, and they require less power to drive their lasing action. Within optical communication systems utilizing VCSELs, it is important to control the optical data signal being transmitted. Because VCSELs which are commonly used in today""s optoelectronics industry, emit a unidirectional light beam normal to the surface in which they are formed, it is especially difficult to monitor the output optical power of the VCSEL which provides the data signal without attenuating or otherwise compromising the integrity of the light emitted. Logically, in order to control the output optical power of a VCSEL and to maintain the optical power at a desired level, the output optical power must first be monitored. Based on the monitored output optical power, adjustments can be made to the current signals supplied to the VCSEL to control the output optical power of the VCSEL which provides the data signal.
Refracted ray coupling techniques, also known as xe2x80x9cRefracted Near Fieldxe2x80x9d techniques, are used in the field of optoelectronics to characterize optical fibers, for example. Such a technique is discussed in xe2x80x9cOptical Fiber Index Profiles by the Refracted-Ray Method (Refracted Near-Field Scanning)xe2x80x9d, Appl. Opt. 20(19), 3415-3421, Oct. 1981, the contents of which are herein incorporated by reference. Optical fibers typically include a core glass section, surrounded by a cladding layer which has a refractive index which is lower than the refractive index of the core layer. When an optical source, such as a VCSEL or other laser, is coupled to an optical fiber, some, and preferably most, of the light is propagated as guided rays in the core of the optical fiber. Light which is not guided along the core escapes into the surrounding cladding layer and is propagated along the cladding layer as cladding rays. Cladding rays quickly become attenuated. According to refracted ray coupling techniques, the cladding layer of the optical fiber is coated with a material such as a laser liquid or a UV-cured liquid, which has a greater refractive index than the cladding layer. According to this technique, some of the cladding mode light exits the cladding layer and therefore the optical fiber, through the coating. The coating essentially draws the rays out of the cladding layer. The angle and power of the light refracted out of the optical fiber is measured and, using Snell""s law, the relative indices of refraction of the core layer and the cladding layers can be determined. Such characterization is commonly performed upon an optical fiber prior to it being installed in the field.
Because of the exhibited need to monitor and control the optical output of a VCSEL, it would be useful to utilize the portion of emitted light from a VCSEL, that exits the optical fiber in accordance with refracted ray coupling principles to provide such monitoring.
The present invention provides an apparatus and method for causing a portion of light emitted by a laser to be refracted out of the optical fiber to which the laser is coupled, and for using the refracted light to monitor the laser. The refracted light exits the optical fiber through a coating formed around the cladding layer which, in turn, surrounds the core of the optical fiber. The light which is refracted out of the optical fiber is directed onto a photodetector that detects the refracted rays representing a portion of the light emitted from the laser.